This invention relates to liquid-metal electrodes for use in electrochemical cells employed for energy storage or electrochemical separations. It has particular applicability to high-temperature, secondary electrochemical cells with electrodes that use the alkali metals such as lithium or sodium, alkaline earth metals such as calcium, or molten alloys of any of these metals as electrode reactants.
A large amount of work has been done in the development of these type electrochemical cells. Both molten and solid alkali metal and alkaline earth metal alloys are contemplated as negative electrode reactants. Positive electrodes in such cells often include as reactants the chalcogens or metal chalcogenides such as the sulfides of iron, nickel, cobalt or copper and mixtures of these materials. Examples of such secondary cells and their various components are illustrated in U.S. Pat. Nos. 3,933,521, Jan. 20, 1976, to Vissers et al.; 3,933,520, Jan. 20, 1976, to Gay et al.; 3,941,612, Mar. 2, 1976, to Steunenberg et al.; and 3,947,291, Mar. 30, 1976 to Yao et al.
Thus far, the most successful lithium electrodes have been those of the solid lithium alloys, particularly the intermetallic lithium-aluminum compounds. Although these solid lithium alloys are simple to fabricate and provide electrode stability, the lithium-aluminum alloys have higher equivalent weight and about 300 mV less anodic potential than elemental lithium metal. Thus, the use of lithium-aluminum alloys entails some sacrifice in energy and power.
Negative electrodes that employ molten elemental lithium as reactant have been constructed to retain the lithium within porous metal substrates such as metal felt, foam or compacted fibers submersed within molten electrolyte. Although wetting of solid metal substrates by liquid metal is improved somewhat by alloying lithium with for instance copper or zinc (U.S. Pat. No. 3,881,951) or by coating the substrate with a metal such as cobalt (U.S. Pat. No. 3,933,521), preferential wetting of the substrate by electrolyte often occurs upon repeated cycling of the electrode. Liquid metal droplets then form, that is dewetting by the liquid metal occurs, which may short circuit the cell.
Therefore, in view of the difficulties encountered in the use of liquid metal electrodes it is an object of the present invention to provide an improved electrode structure that employs molten metal as the electrode reactant.
It is a further object to minimize dewetting between the electrode substrate and the molten metal reactant during cell cycling.
It is also an object to minimize cell shorting from droplets or bridges of molten metal reactant between electrodes.